Remove unused welcome page strings.

TBR=newt
BUG=326023
NOTRY=true

Review URL: https://codereview.chromium.org/165813002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251375 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251375
Propagate engine ID to distinguish the event sender and avoid infinity event loop.

BUG=335406

Review URL: https://codereview.chromium.org/140253008

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251376 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251376